United States of AmericaThe United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, a federal district of Washington, D.C., five major territories, and various possessions. At 3.8 million square miles (9.9 million km2) and with over 320 million people, the country is the world's third largest by total area (fourth largest by land area) and the third most populous.